The need for ever-smaller pumping devices, particularly in the medical microdevice industry, continues to grow. As a result, the need for increasingly small operational pump components, such as check valves, is growing as well, challenging the limits of conventional manufacturing processes. The smallest commonly available check valves have dimensions in the range of 2-10 mm—too large for convenient integration into implantable micropumps with total dimensions in the range of 5-15 mm, as are desirable, e.g., for implantation into small organs such as the eye. Valves less than 1 or 2 mm in size, on the other hand, are difficult to fabricate using conventional technologies.
Part of the challenge in scaling down check valves lies in the complexity of traditional macro-size valve structures. A ball valve, for example, may include a ball, a spring, a rubber valve seal, and a housing fixture. The smoother the surfaces are and the closer the ball is to having perfect spherical shape, the better will be the contact between ball and valve seal, which defines the leakage rate and flow performance of the valve. At small scales, however, surface roughness and shape are difficult to control, and manufacture is, moreover, prone to misalignments of components (e.g., due to crimping of the outer housing). Consequently, it is hard to scale ball valves down in size while retaining proper function and performance. Similarly, silicone valves (e.g., duckbill valves) produced with conventional molding techniques tend to be unreliable (e.g., exhibiting leakage and large production variations) when scaled down to sub-millimeter dimensions. Yet, accurate, repeatable, and reliable flow/pressure performance is critical for many applications, such as drug delivery, where inaccuracies in the flow rate translate into potentially harmful or even fatal under- or overdosing.
A further challenge in the design of microscale check valves is the desired lifetime of the device. A microscale medical device usually requires an operating lifetime of two to ten years; this is especially true for implantable microscale drug-delivery pump systems. However, microscale check valves are prone to stiction or obstruction caused by microscopic particles, tissue growth, or drug sedation; indeed, conventional valve designs often need to balance a trade-off between good valve sealing in the closed state and a sufficiently open fluid path to avoid clogging when the valve is open. If obstructions in the valve occur, the valve may malfunction and exhibit minor symptoms, such as irregular flow performance, or behavior indicative of more serious damage, such as accidental drug overdelivery due to sudden opening of the valve, no delivery of the drug due to obstruction of the valve, or leakage of the pump due to over-pressure for the drug reservoir. These negative effects are generally enhanced with smaller structures and lumina.
Accordingly, there is a need for reliably performing micro-scale check valves and methods for their reproducible manufacture.